Cereals use chemical defences against insects

Science Notes: Plants rely on their toxins not only to fend off caterpillars but also to regulate defence mechanisms against aphids

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A team of scientists from the University of Bern in Switzerland and the Max Planck Institute for Chemical Ecology and partners have characterized multiple functions of benzoxazinoids in wheat.

The toxic form of the substances makes the plant directly resistant to lepidopteran larvae, whereas a less toxic form regulates indirect defence mechanisms against aphids.

Scientists have identified the “switch” between these different functions as a methyltransferase enzyme, which is activated by caterpillar feeding. This switch enables wheat plants to adapt their defence response to different herbivores.

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A comparison with corn shows that a methyltransferase also regulates defence processes in corn against different pest insects.

However, the two enzymes in wheat and corn have evolved independently from each other. The research was published in December in Science Advances.

In nature, plants are exposed to many enemies. In response to these threats, plants have evolved the capacity to produce secondary metabolites whose functions include preventing herbivores from feeding. Plants can use such defensive substances in many different ways.

A team of researchers led by Tobias Köllner from the Max Planck Institute for Chemical Ecology and Matthias Erb from the University of Bern has now characterized how benzoxazinoids function in wheat. The researchers used previously obtained, detailed knowledge about the defensive functions of benzoxazinoids in corn.

In corn plants, the enzyme methyltransferase acts as a functional switch: it decides whether benzoxazinoids act as efficient toxins to protect the plant from caterpillar herbivory, or whether benzoxazinoids are less toxic, but induce callose production. Callose is used as a cell sealant that blocks sieve elements and makes it difficult for the aphids to suck phloem sap.

The approach allowed for a thorough analysis of how switching between toxin production and defence regulation affects wheat resistance to lepidopteran larvae and aphids. Moreover, the scientists were able to identify the corresponding switch in wheat and to analyze it from a biochemical and phylogenetic perspective.

Although corn and wheat both produce benzoxazinoids, in both species, the genes responsible for switching between their toxic and regulative forms are only distantly related.

Thus, the two cereal species likely evolved this switch independently during the course of evolution. Scientists call this phenomenon “convergent evolution.”

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